Toshiko "Tosh" Sato (1975/1981-2009) applied her scientific skills to the Torchwood Three team, after having worked for the Ministry of Defence. Beautiful, brilliant and insecure, much of Tosh's private life remained mysterious even after her death. She lost her life after enduring severe blood loss from a gunshot inflicted by Gray, but before the wound claimed her life, she fought on long enough to help Owen Harper save Cardiff from a nuclear disaster, even though it cost him his life as well.

While covering for teammate Owen Harper, who was suffering from a serious hangover, (TV: Exit Wounds) Tosh, pretending to be a medic named "Dr Sato", performed an autopsy on the supposed pilot of a spaceship crash inside Albion Hospital. The pilot, (TV: Aliens of London) which Tosh nicknamed the "Space Pig", (TV: Exit Wounds) was not dead, but in shock or hibernation. The creature's x-rays were nothing like she had seen before, and Tosh assumed "that's what aliens looked like". The creature, which was still alive, broke free from the morgue and injured Tosh.

After soldiers shot down the shocked creature, the Ninth Doctor explained to Tosh that it was a ordinary Earth pig and "someone's taken [the] pig, opened up its brain, stuck bits on, then they've strapped it in that ship and made it divebomb". Tosh realised from this that the technology couldn't have been from Earth, and the aliens were faking aliens. (TV: Aliens of London)

The Torchwood team travelled to the Welsh countryside to investigate a series of gruesome murders. The Torchwood SUV was stolen by a group of cannibals that harvested travellers once every ten years. The team followed the cannibals to their village. Right as his team were captured, Jack stepped in and incapacitated the cannibals. The cannibals were arrested by the police. (TV: Countrycide)

Tosh had a lesbian affair with "Mary", a female alien who tried to use their relationship to gain access to her technology, which Torchwood had previously confiscated. In order to gain Tosh's confidence, the alien allowed her the use of a pendant which granted her the temporary power of telepathy. Tosh used the device to save the lives of Carol and her child, about to be murdered in a domestic incident, but she became suspicious of the device when its use gave her insight as to how her teammates regarded her. Jack tricked Mary into being teleported into the Sun with her technology. Afterwards, Tosh decided to destroy the pendant, fearing what consequence such a device could have for humanity. (TV: Greeks Bearing Gifts)

Gwen revived Suzie Costello with the resurrection gauntlet for questioning. Tosh was unable to look her former teammate in the eye. Instead of keeping her alive for a minute for a brief interrogation, Suzie kept the link between her and Gwen. After Jack's bullets were useless against Suzie, Tosh was ordered to destroy the gauntlet, which killed Suzie and saved Gwen. (TV: They Keep Killing Suzie)

In 2008, while investigating reports of loud music from the 1940s, the Rift brought Jack and Tosh into 1941. She hid messages that preserved her half of the Rift equations that could bring her home. Owen used these to open up the Rift, bringing Jack and Tosh to the 21st century. (TV: Captain Jack Harkness)

Owen's actions had caused cracks in space and time to spread from the Rift. While cleaning this mess up, Tosh saw an apparition of her mother, telling her to reopen the Rift and undo the damage. The team turned against Jack. Owen shot Jack and reopened the Rift to send everything that had come through back. The opening of the Rift forced out Abaddon, which Jack defeated. (TV: End of Days)

Following Jack's departure, Toshiko was briefly promoted to team leader under Torchwood guidelines as their longest-standing member, but subsequently deferred command to Gwen when she was temporarily rendered mentally unfit to command after exposure to an alien artefact (PROSE: Kaleidoscope).

While chasing a Blowfish alien through Cardiff, Jack saved a family's life and quickly took over Torchwood again. Torchwood met an old lover of Jack's, Captain John Hart, who had came through the Rift searching for some canisters that had also come through. He told Torchwood that they contained radioactive bombs. Tosh tracked down where each of the canisters were. While searching for one of the canisters with Owen, John wounded them and stole the canister. Ianto recovered Tosh and Owen and Tosh found where Gwen had been poisoned so she could be treated. They caught up with John, but the contents of the canisters were actually components of a bomb that latched onto the DNA of the owner of the canisters' murderer. Jack and Owen confused the bomb by injecting John with the DNA of the Torchwood team and safely disposed of it. John forcibly left 21st century Cardiff. (TV: Kiss Kiss Bang Bang)

During one of Tommy Brockless' yearly awakenings, the past and present time shifts began to occur, which the uniformed thought of as "ghosts". To prevent a temporal catastrophe, Tosh and the team saw to it that Tommy returned to 1918, after the couple had a short sexual relationship. (TV: To the Last Man)

She fell in love with Adam Smith when he gave her memories that they were in love and when she was told this she still couldn't bring herself to take the retcon pill until she realised it was the right thing to do. (TV: Adam)

Toshiko was personally responsible for shutting down the Pharm. By that time she had actually taken the initiative to ask Owen, with whom she had feelings, out on a date. She was extremely saddened when Owen was fatally shot in the chest by Aaron Copley. (TV: Reset) After Owen's return to life via the resurrection gauntlet early in 2009, she still maintained hope for a relationship, to Owen's disgust given that his currently-deceased status prevented anything sexual from taking place. (TV: A Day in the Death) Later, however, he seemed less negative about the idea. (TV: Something Borrowed)

At some point during the 2000s, Tosh went to Russia. There, she worked with the KVI to investigate Zone 10. She was given Object One by cosmonaut Anna Volokova, who was given it by the Committee. (AUDIO: Zone 10) Tosh managed to reverse engineer the properties of the object in order to make it give good luck. Whilst using it, she stopped a war in Africa just by hearing about it, and went on a date with Owen, who kissed her. Believing that nobody should have that power, she sent it into another dimension and removed her memories of it using retcon. (AUDIO: The Torchwood Archive)

Toshiko and the rest of the team minus Gwen were investigating an old factory that had detected Rift activity. She was caught up in an explosion of rubble and survived by the help of Gwen but had a broken arm. (TV: Fragments)

Tosh was shot by Gray while communicating via radio with Owen, talking him through his task of preventing a nuclear meltdown at a power station in Cardiff. She talked to Owen about how he would die from the meltdown and they discussed the date they had never gone on. Her wound was fatal and she died in Jack's arms after he arrived back at HQ with Gwen and Ianto. She told them how she had failed to save Owen.

As the team deleted Tosh's profile on the computer system, an automated video message she had recorded played. In it, Tosh thanked Jack for saving her life, and proclaimed her love for Owen and the rest of the team. She reassured them that it was okay that she was gone. (TV: Exit Wounds)

Though dead, Tosh's presence was still felt by the others. When faced with certain death at the hands of the Daleks, who were about to overrun the Hub, Gwen expressed her desire to "go out fighting, like Owen, like Tosh" (TV: The Stolen Earth); she and Ianto were saved by a time-lock defence system Tosh had been working on before her death. The Hub was sealed in a time bubble, preventing a Dalek from entering to finish them off. (TV: Journey's End)

After the 456 incident, Jack revealed that he felt responsible for Tosh's death, along with those of all the other Torchwood operatives who died under his command. (TV: Children of Earth: Day Five)

Tosh was a computer genius and arguably the most intelligent member of the Torchwood team. Despite her intelligence, she was insecure and not very confident in her abilities. Tosh was in love with Owen Harper but because of her insecurity, she found it hard to tell him how she felt. [additional sources needed] Tragically both Tosh and Owen were killed before they could get into a romantic relationship. (TV: Reset, Dead Man Walking, A Day in the Death, Exit Wounds)

The series 1 version of the Torchwood website had correspondence from Tosh in an unsent draft sent to her aunt discovered by Ianto. It was translated from Japanese into English by Ianto. It said that "for the last week", Tosh was with a woman, referring to Mary,"going from a schoolgirl crush on Owen through to a fantastically fulfilling sex life with a complete stranger". She called it "a week of firsts", including: Tosh's first one night stand, lesbian kiss, lesbian "shag", and the first time Tosh was late to work because of sex. [1]

During her first on-screen appearances, in Aliens of London, Tosh was a pathologist called in to autopsy an "alien pilot". When reintroduced in Torchwood, she was a computer specialist. In her final appearance (Exit Wounds), the character would explain this discrepancy by explaining she was filling in for Owen, pretending to be an MD because Owen had a hangover.

Tosh left a resounding impression in the Torchwood universe and in the final episode of Children of Earth Jack admits that he feels responsible for her death. It is assumed her body was destroyed when the Hub was blown up by a bomb.

The name Toshiko means "genius child".

Toshiko was the only known member of Torchwood Three other than Jack Harkness to have met an incarnation of the Doctor. Gwen and Ianto had spoken to the Tenth Doctor through the Sub-Wave Network during Journey's End, but Tosh was the only one to have physically met an incarnation of him.